Reducing latency in video encoding and decoding

ABSTRACT

Techniques and tools for reducing latency in video encoding and decoding by constraining latency due to reordering of video frames, and by indicating the constraint on frame reordering latency with one or more syntax elements that accompany encoded data for the video frames. For example, a real-time communication tool with a video encoder sets a syntax element that indicates a constraint on frame reordering latency, which is consistent with inter-frame dependencies between multiple frames of a video sequence, then outputs the syntax element. A corresponding real-time communication tool with a video decoder receives the syntax element that indicates the constraint on frame reordering latency, determines the constraint on frame reordering latency based on the syntax element, and uses the constraint on frame reordering latency to determine when a reconstructed frame is ready for output (in terms of output order).

RELATED APPLICATION

The present application is a continuation of U.S. patent applicationSer. No. 17/890,398, filed Aug. 18, 2022, which is a continuation ofU.S. patent application Ser. No. 17/143,782, filed Jan. 7, 2021, nowU.S. patent Ser. No. 11/451,830, which is a continuation of U.S. patentapplication Ser. No. 17/009,918, filed Sep. 2, 2020, now U.S. patentSer. No. 10/924,766, which is a continuation of U.S. patent applicationSer. No. 15/963,899, filed Apr. 26, 2018, now U.S. patent Ser. No.10/805,640, which is a continuation of U.S. patent application Ser. No.15/647,962, filed Jul. 12, 2017, now U.S. patent Ser. No. 10/003,824,which is a continuation of U.S. patent application Ser. No. 15/201,160,filed Jul. 1, 2016, now U.S. Pat. No. 9,729,898, which is a continuationof U.S. patent application Ser. No. 14/458,997, filed Aug. 13, 2014, nowU.S. Pat. No. 9,426,495, which is a continuation of U.S. patentapplication Ser. No. 13/270,969, filed Oct. 11, 2011, now U.S. Pat. No.8,837,600, the disclosure of which is hereby incorporated by reference.U.S. patent application Ser. No. 13/270,969 claims the benefit of U.S.Provisional Patent Application No. 61/571,553, filed Jun. 30, 2011, thedisclosure of which is hereby incorporated by reference.

BACKGROUND

Engineers use compression (also called source coding or source encoding)to reduce the bit rate of digital video. Compression decreases the costof storing and transmitting video information by converting theinformation into a lower bit rate form. Decompression (also calleddecoding) reconstructs a version of the original information from thecompressed form. A “codec” is an encoder/decoder system.

Over the last two decades, various video codec standards have beenadopted, including the H.261, H.262 (MPEG-2 or ISO/IEC 13818-2), H.263and H.264 (AVC or ISO/IEC 14496-10) standards and the MPEG-1 (ISO/IEC11172-2), MPEG-4 Visual (ISO/IEC 14496-2) and SMPTE 421M standards. Morerecently, the HEVC standard is under development. A video codec standardtypically defines options for the syntax of an encoded video bitstream,detailing parameters in the bitstream when particular features are usedin encoding and decoding. In many cases, a video codec standard alsoprovides details about the decoding operations a decoder should performto achieve correct results in decoding.

A basic goal of compression is to provide good rate-distortionperformance. So, for a particular bit rate, an encoder attempts toprovide the highest quality of video. Or, for a particular level ofquality/fidelity to the original video, an encoder attempts to providethe lowest bit rate encoded video. In practice, depending on the usescenario, considerations such as encoding time, encoding complexity,encoding resources, decoding time, decoding complexity, decodingresources, overall delay, and/or smoothness in playback also affectdecisions made during encoding and decoding.

For example, consider use scenarios such as video playback from storage,video playback from encoded data streamed over a network connection, andvideo transcoding (from one bit rate to another bit rate, or onestandard to another standard). At the encoder side, such applicationsmay permit off-line encoding that is not at all time-sensitive.Therefore, an encoder can increase encoding time and increase resourcesused during encoding to find the most efficient way to compress video,and thereby improve rate-distortion performance. If a small amount ofdelay is also acceptable at the decoder side, the encoder can furtherimprove rate-distortion performance, e.g., by exploiting inter-picturedependencies from pictures farther ahead in a sequence.

On the other hand, consider use scenarios such as remote desktopconferencing, surveillance video, video telephony and other real-timecommunication scenarios. Such applications are time-sensitive. Lowlatency between recording of input pictures and playback of outputpictures is a key factor in performance. When encoding/decoding toolsadapted for non-real-time communication are applied in real-timecommunication scenarios, overall latency is often unacceptably high. Thedelays that these tools introduce during encoding and decoding mayimprove performance for regular video playback, but they disruptreal-time communication.

SUMMARY

In summary, the detailed description presents techniques and tools forreducing latency in video encoding and decoding. The techniques andtools can reduce latency so as to improve responsiveness in real-timecommunication. For example, the techniques and tools reduce overalllatency by constraining latency due to reordering of video frames, andby indicating the constraint on frame reordering latency with one ormore syntax elements that accompany coded data for the video frames.

According to one aspect of the techniques and tools described herein, atool such as a video encoder, real-time communication tool with a videoencoder, or other tool, sets one or more syntax elements that indicate aconstraint on latency (e.g., a constraint on frame reordering latencyconsistent with inter-frame dependencies between multiple frames of avideo sequence). The tool outputs the syntax element(s), therebyfacilitating simpler and quicker determination of when reconstructedframes are ready for output in terms of output order of the frames.

According to another aspect of the techniques and tools describedherein, a tool such as a video decoder, real-time communication toolwith a video decoder, or other tool, receives and parses one or moresyntax elements that indicate a constraint on latency (e.g., aconstraint on frame reordering latency). The tool also receives encodeddata for multiple frames of a video sequence. At least some of theencoded data is decoded to reconstruct one of the frames. The tool candetermine the constraint on latency based on the syntax element(s), thenuse the constraint on latency to determine when a reconstructed frame isready for output (in terms of output order). The tool outputs thereconstructed frame.

The foregoing and other objects, features, and advantages of theinvention will become more apparent from the following detaileddescription, which proceeds with reference to the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of an example computing system in which somedescribed embodiments can be implemented.

FIGS. 2 a and 2 b are diagrams of example network environments in whichsome described embodiments can be implemented.

FIG. 3 is a diagram of an example encoder system in conjunction withwhich some described embodiments can be implemented.

FIG. 4 is a diagram of an example decoder system in conjunction withwhich some described embodiments can be implemented.

FIGS. 5 a-5 e are diagrams showing coded order and output order forframes in several example series.

FIG. 6 is a flowchart showing an example technique for setting andoutputting one or more syntax elements that indicate a constraint onlatency.

FIG. 7 is a flowchart showing an example technique for reduced-latencydecoding.

DETAILED DESCRIPTION

The detailed description presents techniques and tools for reducinglatency in video encoding and decoding. The techniques and tools canhelp reduce latency so as to improve responsiveness in real-timecommunication.

In video coding/decoding scenarios, some delay is inevitable between thetime an input video frame is received and the time the frame is playedback. The frame is encoded by an encoder, delivered to a decoder anddecoded by the decoder, and some amount of latency is caused bypractical limitations on encoding resources, decoding resources and/ornetwork bandwidth. Other latency is avoidable, however. For example,latency might be introduced by an encoder and decoder to improverate-distortion performance (e.g., to exploit inter-frame dependenciesfrom pictures farther ahead in a sequence). Such latency can be reduced,although there may be a penalty in terms of rate-distortion performance,processor utilization or playback smoothness.

With techniques and tools described herein, latency is decreased byconstraining latency (hence, limiting the temporal extent of inter-framedependencies) and indicating the constraint on latency to a decoder. Forexample, the constraint on latency is a constraint on frame reorderinglatency. Alternatively, the constraint on latency is a constraint interms of seconds, milliseconds, or another time measure. The decoder canthen determine the constraint on latency and use the constraint whendetermining which frames are ready for output. In this way, delay can bereduced for remote desktop conferencing, video telephony, videosurveillance, web camera video and other real-time communicationapplications.

Some of the innovations described herein are illustrated with referenceto syntax elements and operations specific to the H.264 and/or HEVCstandard. Such innovations can also be implemented for other standardsor formats.

More generally, various alternatives to the examples described hereinare possible. Certain techniques described with reference to flowchartdiagrams can be altered by changing the ordering of stages shown in theflowcharts, by splitting, repeating or omitting certain stages, etc. Thevarious aspects of latency reduction for video encoding and decoding canbe used in combination or separately. Different embodiments use one ormore of the described techniques and tools. Some of the techniques andtools described herein address one or more of the problems noted in thebackground. Typically, a given technique/tool does not solve all suchproblems.

I. Example Computing Systems.

FIG. 1 illustrates a generalized example of a suitable computing system(100) in which several of the described techniques and tools may beimplemented. The computing system (100) is not intended to suggest anylimitation as to scope of use or functionality, as the techniques andtools may be implemented in diverse general-purpose or special-purposecomputing systems.

With reference to FIG. 1 , the computing system (100) includes one ormore processing units (110, 115) and memory (120, 125). In FIG. 1 , thismost basic configuration (130) is included within a dashed line. Theprocessing units (110, 115) execute computer-executable instructions. Aprocessing unit can be a general-purpose central processing unit (CPU),processor in an application-specific integrated circuit (ASIC) or anyother type of processor. In a multi-processing system, multipleprocessing units execute computer-executable instructions to increaseprocessing power. For example, FIG. 1 shows a central processing unit(110) as well as a graphics processing unit or co-processing unit (115).The tangible memory (120, 125) may be volatile memory (e.g., registers,cache, RAM), non-volatile memory (e.g., ROM, EEPROM, flash memory,etc.), or some combination of the two, accessible by the processingunit(s). The memory (120, 125) stores software (180) implementing one ormore innovations for reducing latency in video encoding and decoding, inthe form of computer-executable instructions suitable for execution bythe processing unit(s).

A computing system may have additional features. For example, thecomputing system (100) includes storage (140), one or more input devices(150), one or more output devices (160), and one or more communicationconnections (170). An interconnection mechanism (not shown) such as abus, controller, or network interconnects the components of thecomputing system (100). Typically, operating system software (not shown)provides an operating environment for other software executing in thecomputing system (100), and coordinates activities of the components ofthe computing system (100).

The tangible storage (140) may be removable or non-removable, andincludes magnetic disks, magnetic tapes or cassettes, CD-ROMs, DVDs, orany other medium which can be used to store information in anon-transitory way and which can be accessed within the computing system(100). The storage (140) stores instructions for the software (180)implementing one or more innovations for latency reduction in videoencoding and decoding.

The input device(s) (150) may be a touch input device such as akeyboard, mouse, pen, or trackball, a voice input device, a scanningdevice, or another device that provides input to the computing system(100). For video encoding, the input device(s) (150) may be a camera,video card, TV tuner card, or similar device that accepts video input inanalog or digital form, or a CD-ROM or CD-RW that reads video samplesinto the computing system (100). The output device(s) (160) may be adisplay, printer, speaker, CD-writer, or another device that providesoutput from the computing system (100).

The communication connection(s) (170) enable communication over acommunication medium to another computing entity. The communicationmedium conveys information such as computer-executable instructions,audio or video input or output, or other data in a modulated datasignal. A modulated data signal is a signal that has one or more of itscharacteristics set or changed in such a manner as to encode informationin the signal. By way of example, and not limitation, communicationmedia can use an electrical, optical, RF, or other carrier.

The techniques and tools can be described in the general context ofcomputer-readable media. Computer-readable media are any availabletangible media that can be accessed within a computing environment. Byway of example, and not limitation, with the computing system (100),computer-readable media include memory (120, 125), storage (140), andcombinations of any of the above.

The techniques and tools can be described in the general context ofcomputer-executable instructions, such as those included in programmodules, being executed in a computing system on a target real orvirtual processor. Generally, program modules include routines,programs, libraries, objects, classes, components, data structures, etc.that perform particular tasks or implement particular abstract datatypes. The functionality of the program modules may be combined or splitbetween program modules as desired in various embodiments.Computer-executable instructions for program modules may be executedwithin a local or distributed computing system.

The terms “system” and “device” are used interchangeably herein. Unlessthe context clearly indicates otherwise, neither term implies anylimitation on a type of computing system or computing device. Ingeneral, a computing system or computing device can be local ordistributed, and can include any combination of special-purpose hardwareand/or general-purpose hardware with software implementing thefunctionality described herein.

For the sake of presentation, the detailed description uses terms like“determine” and “use” to describe computer operations in a computingsystem. These terms are high-level abstractions for operations performedby a computer, and should not be confused with acts performed by a humanbeing. The actual computer operations corresponding to these terms varydepending on implementation.

II. Example Network Environments.

FIGS. 2 a and 2 b show example network environments (201, 202) thatinclude video encoders (220) and video decoders (270). The encoders(220) and decoders (270) are connected over a network (250) using anappropriate communication protocol. The network (250) can include theInternet or another computer network.

In the network environment (201) shown in FIG. 2 a , each real-timecommunication (“RTC”) tool (210) includes both an encoder (220) and adecoder (270) for bidirectional communication. A given encoder (220) canproduce output compliant with the SMPTE 421M standard, ISO-IEC 14496-10standard (also known as H.264 or AVC), HEVC standard, another standard,or a proprietary format, with a corresponding decoder (270) acceptingencoded data from the encoder (220). The bidirectional communication canbe part of a video conference, video telephone call, or other two-partycommunication scenario. Although the network environment (201) in FIG. 2a includes two real-time communication tools (210), the networkenvironment (201) can instead include three or more real-timecommunication tools (210) that participate in multi-party communication.

A real-time communication tool (210) manages encoding by an encoder(220). FIG. 3 shows an example encoder system (300) that can be includedin the real-time communication tool (210). Alternatively, the real-timecommunication tool (210) uses another encoder system. A real-timecommunication tool (210) also manages decoding by a decoder (270). FIG.4 shows an example decoder system (400), which can be included in thereal-time communication tool (210). Alternatively, the real-timecommunication tool (210) uses another decoder system.

In the network environment (202) shown in FIG. 2 b , an encoding tool(212) includes an encoder (220) that encodes video for delivery tomultiple playback tools (214), which include decoders (270). Theunidirectional communication can be provided for a video surveillancesystem, web camera monitoring system, remote desktop conferencingpresentation or other scenario in which video is encoded and sent fromone location to one or more other locations. Although the networkenvironment (202) in FIG. 2 b includes two playback tools (214), thenetwork environment (202) can include more or fewer playback tools(214). In general, a playback tool (214) communicates with the encodingtool (212) to determine a stream of video for the playback tool (214) toreceive. The playback tool (214) receives the stream, buffers thereceived encoded data for an appropriate period, and begins decoding andplayback.

FIG. 3 shows an example encoder system (300) that can be included in theencoding tool (212). Alternatively, the encoding tool (212) uses anotherencoder system. The encoding tool (212) can also include server-sidecontroller logic for managing connections with one or more playbacktools (214). FIG. 4 shows an example decoder system (400), which can beincluded in the playback tool (214). Alternatively, the playback tool(214) uses another decoder system. A playback tool (214) can alsoinclude client-side controller logic for managing connections with theencoding tool (212).

In some cases, the use of a syntax element to indicate latency (e.g.,frame reordering latency) is specific to a particular standard orformat. For example, encoded data can contain one or more syntaxelements that indicate a constraint on latency as part of the syntax ofan elementary coded video bitstream defined according to the standard orformat, or as defined media metadata relating to the encoded data. Insuch cases, the real-time communication tool (210), encoding tool (212)and/or playback tool (214) with reduced latency may be codec dependent,in that decisions they make can depend on bitstream syntax for aparticular standard or format.

In other cases, the use of a syntax element to indicate a constraint onlatency (e.g., frame reordering latency) is outside a particularstandard or format. For example, syntax element(s) that indicate aconstraint on latency can be signaled as part of the syntax of a mediatransmission stream, media storage file or, more generally, a mediasystem multiplexing protocol or transport protocol. Or, syntaxelement(s) that indicate latency can be negotiated between real-timecommunication tools (210), encoding tools (212) and/or playback tools(214) according to a media property negotiation protocol. In such cases,the real-time communication tool (210), encoding tool (212) and playbacktool (214) with reduced latency may be codec independent, in that theycan work with any available video encoder and decoder, assuming a levelof control over inter-frame dependencies set during encoding.

III. Example Encoder Systems.

FIG. 3 is a block diagram of an example encoder system (300) inconjunction with which some described embodiments may be implemented.The encoder system (300) can be a general-purpose encoding tool capableof operating in any of multiple encoding modes such as a low-latencyencoding mode for real-time communication, transcoding mode, and regularencoding mode for media playback from a file or stream, or it can be aspecial-purpose encoding tool adapted for one such encoding mode. Theencoder system (300) can be implemented as an operating system module,as part of an application library or as a standalone application.Overall, the encoder system (300) receives a sequence of source videoframes (311) from a video source (310) and produces encoded data asoutput to a channel (390). The encoded data output to the channel caninclude one or more syntax elements that indicate a constraint onlatency (e.g., frame reordering latency) to facilitate reduced-latencydecoding.

The video source (310) can be a camera, tuner card, storage media, orother digital video source. The video source (310) produces a sequenceof video frames at a frame rate of, for example, 30 frames per second.As used herein, the term “frame” generally refers to source, coded orreconstructed image data. For progressive video, a frame is aprogressive video frame. For interlaced video, in example embodiments,an interlaced video frame is de-interlaced prior to encoding.Alternatively, two complementary interlaced video fields are encoded asan interlaced video frame or separate fields. Aside from indicating aprogressive video frame, the term “frame” can indicate a singlenon-paired video field, a complementary pair of video fields, a videoobject plane that represents a video object at a given time, or a regionof interest in a larger image. The video object plane or region can bepart of a larger image that includes multiple objects or regions of ascene.

An arriving source frame (311) is stored in a source frame temporarymemory storage area (320) that includes multiple frame buffer storageareas (321, 322, . . . , 32 n). A frame buffer (321, 322, etc.) holdsone source frame in the source frame storage area (320). After one ormore of the source frames (311) have been stored in frame buffers (321,322, etc.), a frame selector (330) periodically selects an individualsource frame from the source frame storage area (320). The order inwhich frames are selected by the frame selector (330) for input to theencoder (340) may differ from the order in which the frames are producedby the video source (310), e.g., a frame may be ahead in order, tofacilitate temporally backward prediction. Before the encoder (340), theencoder system (300) can include a pre-processor (not shown) thatperforms pre-processing (e.g., filtering) of the selected frame (331)before encoding.

The encoder (340) encodes the selected frame (331) to produce a codedframe (341) and also produces memory management control signals (342).If the current frame is not the first frame that has been encoded, whenperforming its encoding process, the encoder (340) may use one or morepreviously encoded/decoded frames (369) that have been stored in adecoded frame temporary memory storage area (360). Such stored decodedframes (369) are used as reference frames for inter-frame prediction ofthe content of the current source frame (331). Generally, the encoder(340) includes multiple encoding modules that perform encoding taskssuch as motion estimation and compensation, frequency transforms,quantization and entropy coding. The exact operations performed by theencoder (340) can vary depending on compression format. The format ofthe output encoded data can be a Windows Media Video format, VC-1format, MPEG-x format (e.g., MPEG-1, MPEG-2, or MPEG-4), H.26x format(e.g., H.261, H.262, H.263, H.264), HEVC format or other format.

The coded frames (341) and memory management control signals (342) areprocessed by a decoding process emulator (350). The decoding processemulator (350) implements some of the functionality of a decoder, forexample, decoding tasks to reconstruct reference frames that are used bythe encoder (340) in motion estimation and compensation. The decodingprocess emulator (350) uses the memory management control signals (342)to determine whether a given coded frame (341) needs to be reconstructedand stored for use as a reference frame in inter-frame prediction ofsubsequent frames to be encoded. If the control signals (342) indicatethat a coded frame (341) needs to be stored, the decoding processemulator (350) models the decoding process that would be conducted by adecoder that receives the coded frame (341) and produces a correspondingdecoded frame (351). In doing so, when the encoder (340) has useddecoded frame(s) (369) that have been stored in the decoded framestorage area (360), the decoding process emulator (350) also uses thedecoded frame(s) (369) from the storage area (360) as part of thedecoding process.

The decoded frame temporary memory storage area (360) includes multipleframe buffer storage areas (361, 362, . . . , 36 n). The decodingprocess emulator (350) uses the memory management control signals (342)to manage the contents of the storage area (360) in order to identifyany frame buffers (361, 362, etc.) with frames that are no longer neededby the encoder (340) for use as reference frames. After modeling thedecoding process, the decoding process emulator (350) stores a newlydecoded frame (351) in a frame buffer (361, 362, etc.) that has beenidentified in this manner.

The coded frames (341) and memory management control signals (342) arealso buffered in a temporary coded data area (370). The coded data thatis aggregated in the coded data area (370) can contain, as part of thesyntax of an elementary coded video bitstream, one or more syntaxelements that indicate a constraint on latency. Or, the coded data thatis aggregated in the coded data area (370) can include syntax element(s)that indicate a constraint on latency as part of media metadata relatingto the coded video data (e.g., as one or more parameters in one or moresupplemental enhancement information (“SEI”) messages or video usabilityinformation (“VUI”) messages).

The aggregated data (371) from the temporary coded data area (370) areprocessed by a channel encoder (380). The channel encoder (380) canpacketize the aggregated data for transmission as a media stream, inwhich case the channel encoder (380) can add, as part of the syntax ofthe media transmission stream, syntax element(s) that indicate aconstraint on latency. Or, the channel encoder (380) can organize theaggregated data for storage as a file, in which case the channel encoder(380) can add, as part of the syntax of the media storage file, syntaxelement(s) that indicate a constraint on latency. Or, more generally,the channel encoder (380) can implement one or more media systemmultiplexing protocols or transport protocols, in which case the channelencoder (380) can add, as part of the syntax of the protocol(s), syntaxelement(s) that indicate a constraint on latency. The channel encoder(380) provides output to a channel (390), which represents storage, acommunications connection, or another channel for the output.

IV. Example Decoder Systems.

FIG. 4 is a block diagram of an example decoder system (400) inconjunction with which some described embodiments may be implemented.The decoder system (400) can be a general-purpose decoding tool capableof operating in any of multiple decoding modes such as a low-latencydecoding mode for real-time communication and regular decoding mode formedia playback from a file or stream, or it can be a special-purposedecoding tool adapted for one such decoding mode. The decoder system(400) can be implemented as an operating system module, as part of anapplication library or as a standalone application. Overall, the decodersystem (400) receives coded data from a channel (410) and producesreconstructed frames as output for an output destination (490). Thecoded data can include one or more syntax elements that indicate aconstraint on latency (e.g., frame reordering latency) to facilitatereduced-latency decoding.

The decoder system (400) includes a channel (410), which can representstorage, a communications connection, or another channel for coded dataas input. The channel (410) produces coded data that has been channelcoded. A channel decoder (420) can process the coded data. For example,the channel decoder (420) de-packetizes data that has been aggregatedfor transmission as a media stream, in which case the channel decoder(420) can parse, as part of the syntax of the media transmission stream,syntax element(s) that indicate a constraint on latency. Or, the channeldecoder (420) separates coded video data that has been aggregated forstorage as a file, in which case the channel decoder (420) can parse, aspart of the syntax of the media storage file, syntax element(s) thatindicate a constraint on latency. Or, more generally, the channeldecoder (420) can implement one or more media system demultiplexingprotocols or transport protocols, in which case the channel decoder(420) can parse, as part of the syntax of the protocol(s), syntaxelement(s) that indicate a constraint on latency.

The coded data (421) that is output from the channel decoder (420) isstored in a temporary coded data area (430) until a sufficient quantityof such data has been received. The coded data (421) includes codedframes (431) and memory management control signals (432). The coded data(421) in the coded data area (430) can contain, as part of the syntax ofan elementary coded video bitstream, one or more syntax elements thatindicate a constraint on latency. Or, the coded data (421) in the codeddata area (430) can include syntax element(s) that indicate a constrainton latency as part of media metadata relating to the encoded video data(e.g., as one or more parameters in one or more SEI messages or VUImessages). In general, the coded data area (430) temporarily storescoded data (421) until such coded data (421) is used by the decoder(450). At that point, coded data for a coded frame (431) and memorymanagement control signals (432) are transferred from the coded dataarea (430) to the decoder (450). As decoding continues, new coded datais added to the coded data area (430) and the oldest coded dataremaining in the coded data area (430) is transferred to the decoder(450).

The decoder (450) periodically decodes a coded frame (431) to produce acorresponding decoded frame (451). As appropriate, when performing itsdecoding process, the decoder (450) may use one or more previouslydecoded frames (469) as reference frames for inter-frame prediction. Thedecoder (450) reads such previously decoded frames (469) from a decodedframe temporary memory storage area (460). Generally, the decoder (450)includes multiple decoding modules that perform decoding tasks such asentropy decoding, inverse quantization, inverse frequency transforms andmotion compensation. The exact operations performed by the decoder (450)can vary depending on compression format.

The decoded frame temporary memory storage area (460) includes multipleframe buffer storage areas (461, 462, . . . , 46 n). The decoded framestorage area (460) is an example of a decoded picture buffer. Thedecoder (450) uses the memory management control signals (432) toidentify a frame buffer (461, 462, etc.) in which it can store a decodedframe (451). The decoder (450) stores the decoded frame (451) in thatframe buffer.

An output sequencer (480) uses the memory management control signals(432) to identify when the next frame to be produced in output order isavailable in the decoded frame storage area (460). To reduce latency ofthe encoding-decoding system, the output sequencer (480) uses syntaxelements that indicate constraints on latency to expedite identificationof frames to be produced in output order. When the next frame (481) tobe produced in output order is available in the decoded frame storagearea (460), it is read by the output sequencer (480) and output to theoutput destination (490) (e.g., display). In general, the order in whichframes are output from the decoded frame storage area (460) by theoutput sequencer (480) may differ from the order in which the frames aredecoded by the decoder (450).

V. Syntax Elements that Facilitate Reduced-latency Encoding andDecoding.

In most video codec systems, the coded order (also called the decodingorder or bitstream order) is the order in which video frames arerepresented in coded data in a bitstream and, hence, processed duringdecoding. The coded order may differ from the order in which the framesare captured by a camera before encoding and differ from the order inwhich decoded frames are displayed, stored, or otherwise output afterdecoding (output order or display order). Reordering of frames relativeto the output order has benefits (primarily in terms of compressioncapability), but it increases the end-to-end latency of encoding anddecoding processes.

Techniques and tools described herein reduce latency due to reorderingof video frames and, by providing information about constraints on thereordering latency to decoder systems, also facilitate latency reductionby the decoder systems. Such latency reduction is useful for manypurposes. For example, it can be used to reduce the time lag that occursin interactive video communication using a video conferencing system, sothat the conversation flow and interactivity of communication betweenremote participants will be more rapid and natural.

A. Approaches to Output Timing and Output Ordering.

According to the H.264 standard, a decoder can use two approaches todetermine when a decoded frame is ready to be output. A decoder can usetiming information in the form of decoding timestamps and outputtimestamps (e.g., as signaled in picture timing SEI messages). Or, thedecoder can use buffering capacity limits signaled with various syntaxelements to determine when a decoded frame is ready to be output.

Timing information can be associated with each decoded frame. Thedecoder can use timing information to determine when a decoded frame canbe output. In practice, however, such timing information may beunavailable to a decoder. Moreover, even when timing information isavailable, some decoders do not actually use this information (e.g.,because a decoder has been designed to work regardless of whether timinginformation is available).

Buffering capacity limits are indicated with several syntax elementsaccording to the H.264 standard (and draft versions of the HEVCstandard), including the syntax element max_dec_frame_buffering, thesyntax element num_reorder_frames, relative ordering information (termed“picture order count” information) and other memory management controlinformation signaled in the bitstream. The syntax elementmax_dec_frame_buffering (or the derived variable specified asMaxDpbFrames) specifies the required size of a decoded picture buffer(“DPB”) in units of frame buffers. As such, the syntax elementmax_dec_frame_buffering expresses a top-level memory capacity used for acoded video sequence, so as to enable a decoder to output pictures inthe correct order. The syntax element num_reorder_frames (ormax_num_reorder_frames) indicates the maximum number of frames (orcomplementary field pairs, or non-paired fields) that can precede anyframe (or complementary field pair, or non-paired field) in coded orderand follow it in output order. In other words, num_reorder_framesspecifies a constraint on the memory capacity necessary for picturereordering. The syntax element max_num_ref_frames specifies the maximumnumber of short-term and long-term reference frames (or complementaryreference field pairs, or non-paired reference fields) that may be usedby the decoding process for inter prediction of any picture in thesequence. The syntax element max_num_ref_frames also determines the sizeof the sliding window for decoded reference picture marking. Likenum_reorder_frames, max_num_ref_frames specifies a constraint onrequired memory capacity.

A decoder uses the max_dec_frame_buffering (or MaxDpbFrames) andnum_reorder_frames syntax elements to determine when a bufferingcapacity limit has been exceeded. This happens, for example, when a newdecoded frame needs to be stored in the DPB, but there is no availablespace remaining in the DPB. In this situation, the decoder uses pictureorder count information to identify, among the pictures that have beendecoded, which is the earliest in output order. The picture that isearliest in output order is then output. Such processing is sometimescalled “bumping” because a picture is “bumped out” of the DPB by thearrival of a new picture that needs to be stored.

Information indicated with the max_dec_frame_buffering (or MaxDpbFrames)and num_reorder_frames syntax elements suffices for determining memorycapacity needed in a decoder. When used to control the “bumping” processfor picture output, however, use of such information can introducelatency unnecessarily. As defined in the H.264 standard, themax_dec_frame_buffering and num_reorder_frames syntax elements do notestablish a limit on the amount of reordering that can be applied to anyparticular picture and, hence, do not establish a limit on end-to-endlatency. Regardless of the values of these syntax elements, a particularpicture can be kept in the DPB for an arbitrarily long time before it isoutput, which corresponds to substantial latency added by pre-bufferingof the source pictures by an encoder.

B. Syntax Elements that Indicate Constraints on Frame ReorderingLatency.

Techniques and tools described herein reduce latency in a videocommunication system. An encoding tool, real-time communication tool, orother tool sets a limit on the extent of reordering that can be appliedto any frame in a coded video sequence. For example, the limit isexpressed as a number of frames that can precede any given frame in acoded video sequence in output order and follow it in coded order. Thelimit constrains the reordering latency allowed for any particular framein the sequence. Stated differently, the limit constrains the temporalextent of reordering (in terms of frames) between coded order and outputorder that can be applied to any particular frame. Limiting the extentof reordering helps reduce end-to-end delay. Also, establishing such alimit can be useful in real-time system negotiation protocols orapplication specifications for use scenarios in which reducing latencyis important.

One or more syntax elements indicate the constraint on frame reorderinglatency. Signaling a constraint on frame reordering latency facilitatessystem-level negotiation for interactive real-time communication orother use scenarios. It provides a way to directly express constraintson frame reordering latency and characterize properties of a mediastream or session.

A video decoder can use an indicated constraint on frame reorderinglatency to enable reduced-latency output of decoded video frames. Inparticular, compared to frame “bumping” processes, signaling aconstraint on frame reordering latency enables a decoder to more simplyand quickly identify frames in a DPB that are ready to be output. Forexample, a decoder can determine the latency status of a frame in theDPB by computing the difference between coded order for the frame andoutput order. By comparing the latency status of the frame to theconstraint on frame reordering latency, a decoder can determine when theconstraint on frame reordering latency has been reached. The decoder canimmediately output any frame that has reached this limit. This can helpthe decoder more rapidly identify frames that are ready for output,compared to “bumping” processes that use a variety of syntax elementsand tracking structures. In this way, the decoder can rapidly (andearlier) determine when a decoded frame can be output. The more rapidly(and earlier) the decoder can identify when frames can be output, themore rapidly (and earlier) the decoder can output video to a display orsubsequent processing stages.

Thus, using the constraint on frame reordering latency, a decoder canbegin output of frames from the decoded frame storage area before thedecoded frame storage area is full, but still provide conformantdecoding (i.e., decoding all frames such that the frames are bit-exactmatches of frames decoded using another, conventional scheme). Thissignificantly reduces delay when the delay (in frames) indicated by thelatency syntax element is much lower than the size (in frames) of thedecoded frame storage area.

FIGS. 5 a-5 e illustrate series (501-505) of frames having differentinter-frame dependencies. The series are characterized by differentvalues for (1) the constraint on the memory capacity necessary forpicture reordering (that is, the number of frame buffers used to storereference frames for purposes of reordering, e.g., as indicated with asyntax element num_reorder_frames, and (2) a constraint on framereordering latency, e.g., as specified by a variable MaxLatencyFrames.In FIGS. 5 a-5 e , for a given frame F_(j) ^(k) the subscript jindicates position of the frame in output order and the superscript kindicates position of the frame in coded order. The frames are shown inoutput order—the output order subscript value increases from left toright. Arrows illustrate inter-frame dependencies for motioncompensation, according to which preceding frames in coded order areused for prediction of subsequent frames in coded order. For simplicity,FIGS. 5 a-5 e show inter-frame dependencies at the frame level (and notat the level of macroblocks, blocks, etc. at which reference frames canchange), and FIGS. 5 a-5 e show at most two frames as reference framesfor a given frame. In practice, in some implementations, differentmacroblocks, blocks, etc. in a given frame can use different referenceframes, and more than two reference frames can be used for the givenframe.

In FIG. 5 a , the series (501) includes nine frames. The last frame F₈ ¹in output order uses the first frame F₀ ⁰ as a reference frame. Theother frames in the series (501) use both the last frame F₈ ¹ and firstframe F₀ ⁰ as reference frames. This means the frame F₀ ⁰ is decodedfirst, followed by the frame F₈ ¹, followed by the frame F₁ ², etc. Inthe series (501) shown in FIG. 5 a , the value of num_reorder_framesis 1. At any point in the processing of the decoder system, among theframes shown in FIG. 5 a , there is only one frame (F₈ ¹) stored in thedecoded frame storage area for reordering purposes. (The first frame F₀⁰ is also used as a reference frame and is stored, but is not stored forreordering purposes. Because the output order for the first frame F₀ ⁰is less than the output order of the intermediate frames, the firstframe F₀ ⁰ is not counted for purposes of num_reorder_frames.) Despitethe low value for num_reorder_frames, the series (501) has a relativelyhigh latency—the value of MaxLatencyFrames is 7. After encoding thefirst frame F₀ ⁰, the encoder waits until it has buffered eight moresource frames before encoding the next frame F₁ ² in output order,because the next frame F₁ ² depends on the last frame F₈ ¹ in the series(501). The value of MaxLatencyFrames is effectively the maximum alloweddifference between the subscript value and the superscript value for anyparticular coded frame.

In FIG. 5 b , the series (502) includes nine frames, as in the series(501) of FIG. 5 a , but the inter-frame dependencies are different.Temporal reordering of frames occurs over short extents. As a result,the series (502) has a much lower latency—the value of MaxLatencyFramesis 1. The value of num_reorder_frames is still 1.

In FIG. 5 c , the series (503) includes ten frames. The longestinter-frame dependency is shorter (in temporal extent) than the longestinter-frame dependency in FIG. 5 a , but longer than the longestinter-frame dependency in FIG. 5 b . The series (503) has the same lowvalue of 1 for num_reorder_frames, and it has a relatively low value of2 for MaxLatencyFrames. The series (503) therefore allows a lowerend-to-end latency than the series (501) of FIG. 5 a , although not aslow as the allowable latency of the series (502) of FIG. 5 b.

In FIG. 5 d , the series (504) includes frames organized in a temporalhierarchy with three temporal layers according to inter-framedependencies. The lowest temporal resolution layer includes the firstframe F₀ ⁰ and last frame F₈ ¹. The next temporal resolution layer addsthe frame F₄ ², which depends on the first frame F₀ ⁰ and last frame F₈¹. The highest temporal resolution layer adds the remaining frames. Theseries (504) shown in FIG. 5 d has a relatively low value of 2 fornum_reorder_frames but a relatively high value of 7 forMaxLatencyFrames, at least for the highest temporal resolution layer,due to the difference between coded order and output order for the lastframe F₈ ¹. If only the intermediate temporal resolution layer or lowesttemporal resolution layer is decoded, the constraint on frame reorderingdelay can be reduced to 1 (for the intermediate layer) or 0 (for thelowest layer). To facilitate reduced-latency decoding at varioustemporal resolutions, syntax elements can indicate constraints on framereordering latency for different layers in a temporal hierarchy.

In FIG. 5 e , the series (505) includes frames organized in a temporalhierarchy with three temporal layers according to different inter-framedependencies. The lowest temporal resolution layer includes the firstframe F₀ ⁰, middle frame F₄ ¹ and last frame F₈ ⁵. The next temporalresolution layer adds the frames F₂ ² (which depends on the first frameF₀ ⁰ and middle frame F₄ ¹) and F₆ ⁶ (which depends on the middle frameF₄ ¹ and last frame F₈ ⁵). The highest temporal resolution layer addsthe remaining frames. Compared to the series (504) of FIG. 5 d , theseries (505) of FIG. 5 e still has a relatively low value of 2 fornum_reorder_frames but has a lower value of 3 for MaxLatencyFrames, atleast for the highest temporal resolution layer, due to the differencebetween coded order and output order for the middle frame F₄ ¹ and lastframe F₈ ⁵. If only the intermediate temporal resolution layer or lowesttemporal resolution layer is decoded, the constraint on frame reorderingdelay can be reduced to 1 (for the intermediate layer) or 0 (for thelowest layer).

In the examples shown in FIGS. 5 a-5 e , if the value ofMaxLatencyFrames is known, a decoder can identify certain frames asbeing ready for immediate output upon receipt of the preceding frame inoutput order. For a given frame, the frame's output order value minusthe frame's coded order value may be equal to the value ofMaxLatencyFrames. In this case, the given frame is ready for output assoon as the frame preceding it in output order is received. (Incontrast, such frames could not be identified as ready for output usingnum_reorder_frames alone until additional frames were received or theend of the sequence was reached.) In particular, a decoder can use thevalue of MaxLatencyFrames to enable earlier output of the followingframes:

-   -   In the series (501) of FIG. 5 a , the frame F₈ ¹.    -   In the series (502) of FIG. 5 b , the frames F₂ ¹, F₄ ³, F₆ ⁵,        and F₈ ⁷.    -   In the series (503) of FIG. 5 c , the frames F₃ ¹, F₆ ⁴, and F₉        ⁷.    -   In the series (504) of FIG. 5 d , the frame F₈ ¹.    -   In the series (505) of FIG. 5 e , the frames F₄ ¹, and F₈ ⁵.        In addition, declaration or negotiation of the value of        MaxLatencyFrames at the system level can provide a summary        expression of the latency characteristics of the bitstream or        session in a way that is not enabled by measurement of        reordering storage capacity and indication of such capacity        using num_reorder_frames.

C. Example Implementations.

Syntax elements that indicate a constraint on frame reordering latencycan be signaled in various ways, depending on implementation. The syntaxelements can be signaled as part of a sequence parameter set (“SPS”),picture parameter set (“PPS”), or other element of the bitstream,signaled as part of SEI messages, VUI messages or other metadata, orsignaled in some other way. In any of the implementations, a syntaxelement indicating a constraint value can be encoded using unsignedexponential-Golomb coding, some other form of entropy coding, or fixedlength coding, then signaled. A decoder performs corresponding decodingafter receiving the syntax element.

In a first implementation, a flag max_latency_limitation_flag issignaled. If the flag has a first binary value (e.g., 0), no constrainton frame reordering latency is imposed. In this case, the value of amax_latency_frames syntax element is not signaled or is ignored.Otherwise (the flag having a second binary value such as 1), the valueof a max_latency_frames syntax element is signaled to indicate theconstraint on frame reordering latency. For example, in this case, thevalue signaled for the max_latency_frames syntax element can be anynon-negative integer value.

In a second implementation, a syntax element max_latency_frames_plus1 issignaled to indicate the constraint on frame reordering latency. Ifmax_latency_frames_plus1 has a first value (e.g., 0), no constraint onframe reordering latency is imposed. For other values (e.g., non-zerovalues), the value of the constraint on frame reordering latency is setto max_latency_frames_plus1−1. For example, the value ofmax_latency_frames_plus1 is in the range of 0 to 2³²−2, inclusive.

Similarly, in a third implementation, a syntax elementmax_latency_frames is signaled to indicate the constraint on framereordering latency. If max_latency_frames has a first value (e.g., amaximum value), no constraint on frame reordering latency is imposed.For other values (e.g., values less than the maximum value), the valueof the constraint on frame reordering latency is set tomax_latency_frames.

In a fourth implementation, the constraint on frame reordering latencyis indicated relative to a maximum size of frame memory. For example,the latency constraint is signaled as an increase relative to thenum_reorder_frames syntax element. Ordinarily, the constraint on framereordering latency (in terms of frames) is greater than or equal tonum_reorder_frames. To save bits in signaling of the latency constraint,the difference between the latency constraint and num_reorder_frames isencoded (e.g., using unsigned exponential-Golomb coding, some other formof entropy coding) then signaled. A syntax elementmax_latency_increase_plus1 is signaled to indicate the constraint onframe reordering latency. If max_latency_increase_plus1 has a firstvalue (e.g., 0), no constraint on frame reordering latency is imposed.For other values (e.g., non-zero values), the value of the constraint onframe reordering latency is set tonum_reorder_frames+max_latency_increase_plus1−1. For example, the valueof max_latency_increase_plus1 is in the range of 0 to 2³²−2, inclusive.

Alternatively, one or more syntax elements that indicate a constraint onframe reordering latency are signaled in some other way.

D. Other Ways of Indicating Constraints on Latency.

In many of the preceding examples, the constraint on latency is aconstraint on frame reordering latency that is expressed in terms of acount of frames. More generally, the constraint on latency is aconstraint on delay that can be expressed in terms of a frame count orexpressed in terms of seconds, milliseconds or another time measure. Forexample, the constraint on latency can be expressed as an absolute timemeasure such as 1 second or 0.5 seconds. An encoder can convert such atime measure to a count of frames (considering the frame rate of video),then encode the video such that inter-frame dependencies betweenmultiple frames of a video sequence are consistent with the frame count.Or, regardless of frame reordering and inter-frame dependencies, theencoder can use the time measure to limit the extent to which delay isused to smooth out short-term fluctuations in bit rate of encoded video,encoding complexity, network bandwidth, etc. A decoder can use the timemeasure to determine when a frame can be output from a decoded picturebuffer.

The constraint on latency can be negotiated between a transmitter sideand receiver side so as to trade-off responsiveness (lack of delay) withability to smooth out short-term fluctuations in bit rate of encodedvideo, ability to smooth out short-term fluctuations in encodingcomplexity, ability to smooth out short-term fluctuations in networkbandwidth and/or another factor that benefits from increased delay. Insuch negotiations, it may be helpful to establish and characterize theconstraint on latency in a way that is independent of frame rate. Theconstraint can then be applied during encoding and decoding, consideringthe frame rate of the video. Or, the constraint can be applied duringencoding and decoding regardless of the frame rate of the video.

E. Generalized Techniques for Setting and Outputting Syntax Elements.

FIG. 6 shows an example technique (600) for setting and outputtingsyntax elements that facilitate reduced-latency decoding. For example, areal-time communication tool or encoding tool described with referenceto FIGS. 2 a and 2 b performs the technique (600). Alternatively,another tool performs the technique (600).

To start, the tool sets (610) one or more syntax elements that indicatea constraint on latency (e.g., frame reordering latency, latency interms of a time measure) consistent with inter-frame dependenciesbetween multiple frames of a video sequence. When the tool includes avideo encoder, the same tool can also receive the frames, encode theframes to produce encoded data (using the inter-frame dependencies thatare consistent with the constraint on frame reordering latency), andoutput the encoded data for storage or transmission.

Typically, a constraint on frame reordering latency is a reorderinglatency allowed for any frame in the video sequence. The constraint canbe expressed in various ways, however, and have various other meanings.For example, the constraint can be expressed in terms of a maximum countof frames that can precede a given frame in output order but follow thegiven frame in coded order. Or, the constraint can be expressed as amaximum difference between coded order and output order for any frame inthe video sequence. Or, focusing on an individual frame, the constraintcan be expressed as the reordering latency associated with a given,specific frame in the video sequence. Or, focusing on a group of frames,the constraint can be expressed as the reordering latency associatedwith the group of frames in the video sequence. Or, the constraint canbe expressed in some other way.

Next, the tool outputs (620) the syntax element(s). This facilitatesdetermination of when reconstructed frames are ready for output in termsof output order of the multiple frames. The syntax element(s) can beoutput as part of a sequence parameter set or picture parameter set inan elementary coded video bitstream, as part of syntax for a mediastorage file or media transmission stream that also includes encodeddata for the frames, as part of a media properties negotiation protocol(e.g., during exchange of stream or session parameter values insystem-level negotiation), as part of media system informationmultiplexed with encoded data for the frames, or as part of mediametadata relating to encoded data for the frames (e.g., in SEI messagesor VUI messages). Different syntax elements can be output to indicatememory capacity requirements. For example, a buffer size syntax element(such as max_dec_frame_buffering) can indicate maximum size of a DPB,and a frame memory syntax element (such as num_reorder_frames) canindicate maximum size of frame memory for reordering.

The value of the constraint on latency can be represented in variousways, as described in section V.C. For example, the tool outputs a flagthat indicates presence or absence of the syntax element(s). If the flagindicates the syntax element(s) are absent, then the constraint onlatency is undefined or has a default value. Otherwise, the syntaxelement(s) follow and indicate the constraint on latency. Or, one valueof the syntax element(s) indicates the constraint on latency isundefined or has a default value, and other possible values of thesyntax element(s) indicate an integer count for the constraint onlatency. Or, for cases in which the constraint on latency is aconstraint on frame reordering latency, a given value of the syntaxelement(s) indicates an integer count for the constraint on framereordering latency relative to a maximum size of frame memory forreordering, which is indicated with a different syntax element such asnum_reorder_frames. Alternatively, the constraint on latency isrepresented in some other way.

In some implementations, the frames of the video sequence are organizedaccording to a temporal hierarchy. In this case, different syntaxelements can indicate different constraints on frame reorderinglatencies for different temporal layers of the temporal hierarchy.

F. Generalized Techniques for Receiving and Using Syntax Elements.

FIG. 7 shows an example technique (700) for receiving and using syntaxelements that facilitate reduced-latency decoding. For example, areal-time communication tool or playback tool described with referenceto FIGS. 2 a and 2 b performs the technique (700). Alternatively,another tool performs the technique (700).

To start, the tool receives and parses (710) one or more syntax elementsthat indicate a constraint on latency (e.g., frame reordering latency,latency in terms of a time measure). For example, the parsing includesreading the one or more syntax elements that indicate the constraint onlatency from the bitstream. The tool also receives (720) encoded datafor multiple frames of a video sequence. The tool can parse the syntaxelement(s) and, based upon syntax element(s), determine the constrainton latency. Typically, a constraint on frame reordering latency is areordering latency allowed for any frame in the video sequence. Theconstraint can be expressed in various ways, however, and have variousother meanings, as described in the previous section. The syntaxelement(s) can be signaled as part of a sequence parameter set orpicture parameter set in an elementary coded video bitstream, as part ofsyntax for a media storage file or media transmission stream, as part ofa media properties negotiation protocol, as part of media systeminformation multiplexed with the encoded data, or as part of mediametadata relating to the encoded data. The tool can receive and parsedifferent syntax elements which indicate memory capacity requirements,for example, a buffer size syntax element such asmax_dec_frame_buffering and a frame memory syntax element such asnum_reorder_frames.

The value of the constraint on latency can be represented in variousways, as described in section V.C. For example, the tool receives a flagthat indicates presence or absence of the syntax element(s). If the flagindicates the syntax element(s) are absent, then the constraint onlatency is undefined or has a default value. Otherwise, the syntaxelement(s) follow and indicate the constraint on latency. Or, one valueof the syntax element(s) indicates the constraint on latency isundefined or has a default value, and other possible values of thesyntax element(s) indicate an integer count for the constraint onlatency. Or, for cases in which the constraint on latency is aconstraint on frame reordering latency a given value of the syntaxelement(s) indicates an integer count for the constraint on framereordering latency relative to a maximum size of frame memory forreordering, which is indicated with a different syntax element such asnum_reorder_frames. Alternatively, the constraint on latency is signaledin some other way.

Returning to FIG. 7 , the tool decodes (730) at least some of theencoded data to reconstruct one of the frames. The tool outputs (740)the reconstructed frame. In doing so, the tool can use the constraint onlatency to determine when the reconstructed frame is ready for output,e.g., in terms of output order of the frames of the video sequence.

In some implementations, the frames of the video sequence are organizedaccording to a temporal hierarchy. In this case, different syntaxelements can indicate different constraints on frame reorderinglatencies for different temporal layers of the temporal hierarchy. Thetool can select one of the different constraints on frame reorderinglatencies depending on temporal resolution of the output.

In view of the many possible embodiments to which the principles of thedisclosed invention may be applied, it should be recognized that theillustrated embodiments are only preferred examples of the invention andshould not be taken as limiting the scope of the invention. Rather, thescope of the invention is defined by the following claims. I thereforeclaim as my invention all that comes within the scope and spirit ofthese claims.

I claim:
 1. A computer system comprising one or more processing unitsand memory, wherein the computer system implements a video decoderconfigured to perform operations comprising: receiving and parsing oneor more syntax elements that indicate a constraint on frame reorderinglatency, wherein the constraint on frame reordering latency is expressedin terms of a maximum count of frames that can precede any frame of avideo sequence in output order but follow that frame in coded order;determining the constraint on frame reordering latency based upon theone or more syntax elements; receiving encoded data for a given frame ofthe video sequence; decoding the encoded data for the given frame toreconstruct the given frame; and outputting the reconstructed frame,wherein the constraint is used to determine when the reconstructed frameis ready for output.
 2. The computer system of claim 1, wherein the oneor more syntax elements and the encoded data for the given frame aresignaled as part of syntax for a coded video bitstream, the operationsfurther comprising: receiving and parsing a buffer size syntax elementthat indicates maximum size of a decoded picture buffer, wherein thebuffer size syntax element is different than the one or more syntaxelements that indicate the constraint on frame reordering latency. 3.The computer system of claim 1, wherein the one or more syntax elementsare signaled as part of a sequence parameter set.
 4. The computer systemof claim 1, wherein the operations further comprise: receiving a flagthat indicates presence or absence of the one or more syntax elements.5. The computer system of claim 1, wherein one possible value of the oneor more syntax elements indicates the constraint on frame reorderinglatency is undefined or has a default value, and wherein other possiblevalues of the one or more syntax elements indicate an integer count forthe constraint on frame reordering latency.
 6. The computer system ofclaim 1, wherein the video sequence includes multiple frames organizedaccording to a temporal hierarchy, the multiple frames including thegiven frame, and wherein different constraints on frame reorderinglatencies apply for different temporal layers of the temporal hierarchy.7. The computer system of claim 6, wherein the operations furthercomprise selecting, as the constraint on frame reordering latency, oneof the different constraints on frame reordering latencies depending ontemporal resolution of the output.
 8. The computer system of claim 1,wherein a value of the one or more syntax elements indicates an integercount for the constraint on frame reordering latency relative to amaximum size of frame memory for reordering, the maximum size of framememory for reordering being indicated with a different syntax element.9. The computer system of claim 8, wherein the constraint on framereordering latency can be determined as a maximum count for the maximumsize of frame memory for reordering plus the integer count for theconstraint on frame reordering latency minus
 1. 10. In a computer systemthat implements a video encoder, a method comprising: setting one ormore syntax elements that indicate a constraint on frame reorderinglatency, wherein the constraint on frame reordering latency is expressedin terms of a maximum count of frames that can precede any frame of avideo sequence in output order but follow that frame in coded order;outputting the one or more syntax elements, thereby facilitatingdetermination of when reconstructed frames are ready for output;encoding a given frame of the video sequence, thereby producing encodeddata for the given frame, wherein the encoding uses inter-framedependencies, within the video sequence, that are consistent with theconstraint on frame reordering latency; and outputting the encoded datafor the given frame.
 11. The method of claim 10, wherein the one or moresyntax elements and the encoded data for the given frame are output aspart of syntax for a coded video bitstream, the method furthercomprising: outputting a buffer size syntax element that indicatesmaximum size of a decoded picture buffer, wherein the buffer size syntaxelement is different than the one or more syntax elements that indicatethe constraint on frame reordering latency.
 12. The method of claim 10,wherein the one or more syntax elements are output as part of a sequenceparameter set.
 13. The method of claim 10, wherein one possible value ofthe one or more syntax elements indicates the constraint on framereordering latency is undefined or has a default value, and whereinother possible values of the one or more syntax elements indicate aninteger count for the constraint on frame reordering latency.
 14. Themethod of claim 10, wherein a value of the one or more syntax elementsindicate an integer count for the constraint on frame reordering latencyrelative to a maximum size of frame memory for reordering, the maximumsize of frame memory for reordering being indicated with a differentsyntax element.
 15. The method of claim 14, wherein the constraint onframe reordering latency can be determined as a maximum count for themaximum size of frame memory for reordering plus the integer count forthe constraint on frame reordering latency minus
 1. 16.Computer-readable tangible storage having stored thereon, in abitstream, one or more syntax elements that indicate a constraint onframe reordering latency, wherein the constraint on frame reorderinglatency is expressed in terms of a maximum count of frames that canprecede any frame of a video sequence in output order but follow thatframe in coded order, the computer-readable tangible storage furtherhaving stored thereon, in the bitstream, encoded data for a given frameof the video sequence, the bitstream being organized to facilitateplayback, in a computer system that implements a video decoder, byoperations comprising: parsing the one or more syntax elements thatindicate the constraint on frame reordering latency; determining theconstraint on frame reordering latency based upon the one or more syntaxelements; decoding the encoded data for the given frame to reconstructthe given frame; and outputting the reconstructed frame, wherein theconstraint is used to determine when the reconstructed frame is readyfor output.
 17. The computer-readable tangible storage of claim 16,wherein the one or more syntax elements are signaled as part of asequence parameter set.
 18. The computer-readable tangible storage ofclaim 16, wherein one possible value of the one or more syntax elementsindicates the constraint on frame reordering latency is undefined or hasa default value, and wherein other possible values of the one or moresyntax elements indicate an integer count for the constraint on framereordering latency.
 19. The computer-readable tangible storage of claim16, wherein a value of the one or more syntax elements indicates aninteger count for the constraint on frame reordering latency relative toa maximum size of frame memory for reordering, the maximum size of framememory for reordering being indicated with a different syntax element.20. The computer-readable tangible storage of claim 19, wherein theconstraint on frame reordering latency can be determined as a maximumcount for the maximum size of frame memory for reordering plus theinteger count for the constraint on frame reordering latency minus 1.